1. Field of the Invention
This invention relates to a method and a system for online ferroresonance detection, especially of power transformer ferroresonance.
2. Description of the Related Art
Ferroresonance is a phenomenon that is the occurrence of an unstable high voltage, typically on three phase electrical systems, which only occurs under specific conditions.
Ferroresonance is a very dangerous phenomen for transformer feeder or mesh corner and tee connection constructions where there is a double overhead line section.
Indeed, when a transformer feeder is disconnected from the rest of a power system, the transformer may be driven into saturation due to discharge of the capacitance-to-earth of the isolated system. Ferroresonance may then occur between the reactive components, said ferroresonance being maintained by energy transferred from the coupling capacitance of the parallel line which remains on load. When there is ferroresonance, the re-energized transformer can cause severe switching overvoltages. Therefore a ferroresonance detection and alarm device is essential.
Ferroresonance is a complicated nonlinear electrical resonant phenomenon, which is caused by saturable inductance of a transformer coupling with system capacitance. This phenomenon, which can take place for a wide range of situations in power systems, is very dangerous for power systems due to overvoltages, overcurrents and the abnormal rate of harmonics it bring about, which may cause dielectric and thermal destructions, reduction in performance and lifetime of insulators, failure of the equipment (e.g. untimely tripping of the protection devices), premature ageing of the electrical equipments, even breakdown of whole system.
The main characteristic of ferroresonance is that it is highly sensitive to system parameters and initial conditions, which makes it is hard to be predicted.
There are four different modes of ferroresonance according to the shape and frequency of its voltage, said modes are the fundamental mode, the subharmonic mode, the quasi-periodic mode and the chaotic mode. The fundamental and subharmonic modes are more frequent than the other two in power system.
Conventional UK practice has been to fit ferroresonance detection which automatically initiates isolation of the transformer from the de-energized line by operation of an open terminal disconnector at the onset of ferroresonance: when de-energized, if two out of three phases voltages remain high, the alarm will be issued.
The document referenced [1] at the end of the description describes a protection relay (XR 309) made by the Reyrolle company. On supergrid systems, ferroresonance may be experienced following de-energisation of a directly connected transformer. Ferroresonance may be sustained by the induction from an energized parallel circuit. Re-energising the transformer whilst in a ferroresonant state can risk severe switching overvoltages, therefore where there is such a risk a ferroresonance alarm relay is essential. So the relay XR 309 detect ferroresonance, with the system energized or de-energised, as follows:                On system de-energisation, the secondary voltage falls below the reset level, and three elements drop-off. In the event of ferroresonance occurring, two out of three elements will remain energized.        If ferroresonance is induced onto a de-energised system, the relay will only respond if the amplitude of ferroresonance is above the relay element pick-up level of 40V AC.        Relay contacts are wired to initiate a timer, which in turn will initiate the alarm.        
This prior art method cannot cover all ferroresonance situations: for example the only phase high voltage case in the electrical rail circuits. Another shortcoming of this relay is that it is not numerical but analog. Therefore it cannot be incorporated into the new protection relays.
The invention is related to the detection of ferroresonance and determination of the mode of said ferroresonance, especially in transformer feeder connection conditions, or equivalent, such as mesh corner and circuit tee connections, where a section of double circuit overhead lines exists.
The purpose of the invention is to obtain an accurate detection and mode recognition of ferroresonance, in focusing on its most distinctive feature, which is transformer iron core saturation, and its spectrum performance.